Various kinds of photosensitive lithographic printing plates requiring no fountain solution (hereinafter referred to as "water-free lithographic printing plate") for lithographic printing without fountain solution have been proposed, for example, in JP-B-44-23042, JP-B-46-16044, JP-B-54-26923, JP-B-61-54222, and JP-A-63-265247. (The terms "JP-B" and "JP-A" as used herein mean an "examined Japanese patent publication" and an "unexamined published Japanese patent application," respectively.) Of these, the lithographic printing plates comprising a photosensitive layer and an ink-repellent layer formed in this order on a substrate have extremely high performances.
The substrate for a water-free lithographic printing plate should have flexibility so as to be set on an ordinary lithographic printing machine and also have durability so as to withstand load during printing. Aluminum is the representative example of the material of widely used substrates. In general, a primer layer is formed for the purposes of improving adhesion between the substrate and the photosensitive layer and improving scratch resistance. Examples of the primer layer include a layer formed, prior to the formation of a photosensitive layer, by applying any of various photosensitive polymers such as those disclosed in JP-A-60-229031 and exposing the applied polymer to cure the same; a layer comprising the epoxy resin disclosed in JP-B-61-54219; a layer formed by curing the gelatin disclosed in JP-A-63-133151; a layer comprising the urethane resin and silane coupling agent disclosed in JP-A-3-200965; and a layer comprising the urethane resin disclosed in JP-A-3-273248.
A technique for improving suitability for plate inspection is known in which technique A filler such as titanium oxide is incorporated as a whitening agent into the primer layer. Although the lithographic printing plate thus obtained has improved suitability for visual plate inspection, this technique has a disadvantage in that when the plate is examined with a plate scanner to read the percentage of image area (this reading is necessary for determining ink amount according to the dot percent in the plate), this examination is affected by the rolling grain of the aluminum plate, so that the measured value of image area percentage varies depending on the scanning direction relative to the rolling grain.
For obtaining a water-free lithographic printing plate having suitability for plate scanner examination, a technique of employing a special rolled aluminum plate having high specular gloss and a central mean surface roughness (R.sub.a) of 0.1 .mu.m or less is disclosed in JP-A-4-91992. However, this technique is disadvantageous in that the lithographic printing plate produced is costly due to the use of the special aluminum plate, tends to suffer peeling of the primer layer or photosensitive layer from the substrate during development, and has insufficient press life. In JP-A-55-113595 is disclosed a water-free lithographic printing plate employing an aluminum substrate which has undergone alkali etching or electrolytic etching. This lithographic printing plate however has a drawback of poor tone reproducibility, although improved in both of suitability for visual plate inspection and suitability for plate scanner examination.